1.. _active_mm:
2
3=========
4Active MM
5=========
6
7::
8
9 List:       linux-kernel
10 Subject:    Re: active_mm
11 From:       Linus Torvalds <torvalds () transmeta ! com>
12 Date:       1999-07-30 21:36:24
13
14 Cc'd to linux-kernel, because I don't write explanations all that often,
15 and when I do I feel better about more people reading them.
16
17 On Fri, 30 Jul 1999, David Mosberger wrote:
18 >
19 > Is there a brief description someplace on how "mm" vs. "active_mm" in
20 > the task_struct are supposed to be used?  (My apologies if this was
21 > discussed on the mailing lists---I just returned from vacation and
22 > wasn't able to follow linux-kernel for a while).
23
24 Basically, the new setup is:
25
26  - we have "real address spaces" and "anonymous address spaces". The
27    difference is that an anonymous address space doesn't care about the
28    user-level page tables at all, so when we do a context switch into an
29    anonymous address space we just leave the previous address space
30    active.
31
32    The obvious use for a "anonymous address space" is any thread that
33    doesn't need any user mappings - all kernel threads basically fall into
34    this category, but even "real" threads can temporarily say that for
35    some amount of time they are not going to be interested in user space,
36    and that the scheduler might as well try to avoid wasting time on
37    switching the VM state around. Currently only the old-style bdflush
38    sync does that.
39
40  - "tsk->mm" points to the "real address space". For an anonymous process,
41    tsk->mm will be NULL, for the logical reason that an anonymous process
42    really doesn't _have_ a real address space at all.
43
44  - however, we obviously need to keep track of which address space we
45    "stole" for such an anonymous user. For that, we have "tsk->active_mm",
46    which shows what the currently active address space is.
47
48    The rule is that for a process with a real address space (ie tsk->mm is
49    non-NULL) the active_mm obviously always has to be the same as the real
50    one.
51
52    For a anonymous process, tsk->mm == NULL, and tsk->active_mm is the
53    "borrowed" mm while the anonymous process is running. When the
54    anonymous process gets scheduled away, the borrowed address space is
55    returned and cleared.
56
57 To support all that, the "struct mm_struct" now has two counters: a
58 "mm_users" counter that is how many "real address space users" there are,
59 and a "mm_count" counter that is the number of "lazy" users (ie anonymous
60 users) plus one if there are any real users.
61
62 Usually there is at least one real user, but it could be that the real
63 user exited on another CPU while a lazy user was still active, so you do
64 actually get cases where you have a address space that is _only_ used by
65 lazy users. That is often a short-lived state, because once that thread
66 gets scheduled away in favour of a real thread, the "zombie" mm gets
67 released because "mm_count" becomes zero.
68
69 Also, a new rule is that _nobody_ ever has "init_mm" as a real MM any
70 more. "init_mm" should be considered just a "lazy context when no other
71 context is available", and in fact it is mainly used just at bootup when
72 no real VM has yet been created. So code that used to check
73
74 	if (current->mm == &init_mm)
75
76 should generally just do
77
78 	if (!current->mm)
79
80 instead (which makes more sense anyway - the test is basically one of "do
81 we have a user context", and is generally done by the page fault handler
82 and things like that).
83
84 Anyway, I put a pre-patch-2.3.13-1 on ftp.kernel.org just a moment ago,
85 because it slightly changes the interfaces to accommodate the alpha (who
86 would have thought it, but the alpha actually ends up having one of the
87 ugliest context switch codes - unlike the other architectures where the MM
88 and register state is separate, the alpha PALcode joins the two, and you
89 need to switch both together).
90
91 (From http://marc.info/?l=linux-kernel&m=93337278602211&w=2)
92